High-power, high-throughput microwave discharge singlet oxygen generator for advanced electrical oxygen-iodine lasers

ABSTRACT

A laser device includes an optical resonator, a microwave driven discharge device, and a source for a second gas. The microwave driven discharge device is disposed relative to the optical resonator. The microwave driven discharge device operates at a discharge power and gas flow rate to produce a selected amount of energetic singlet oxygen metastables flowing in the direction of the optical resonator. The second source for the second gas is disposed between the optical resonator and the microwave driven discharge device. The second gas reacts with the selected amount of energetic singlet oxygen metastables to form an excited species in an amount sufficient to support lasing of the excited species in the optical resonator.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 60/881,750, filed Jan. 22, 2007, the entiredisclosure of which is incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with government support under Contract No.FA9451-04-M-0239, awarded by the United States Air Force. The governmentmay have certain rights in the invention.

FIELD OF THE INVENTION

The invention relates generally to a method and apparatus for generatingsinglet oxygen in an enclosed container, and more particularly to amethod and apparatus for efficiently generating energetic singlet oxygenmetastables, at a high discharge power and in a high-throughput gasflow, e.g., for use in Electrical Oxygen-Iodine Laser (EOIL) systems.

BACKGROUND OF THE INVENTION

The Electric Oxygen-Iodine Laser (EOIL) is an emerging concept for acompact, closed-cycle, all-gas-phase, energy transfer laser withhigh-power military and industrial applications. The EOIL uses anelectric discharge of a flowing oxygen gas mixture to generate singletoxygen metastables, O₂(a¹Δ_(g)), and atomic oxygen, which subsequentlyreact with molecular iodine to excite the atomic iodine lasingtransition, I(²P_(1/2)→²P_(3/2)), at 1.315 μm. The viability of EOIL hasbeen recently demonstrated through measurements of positive gain andlasing in low-power laboratory systems. The I(²P_(1/2)) (or I*)excitation mechanism in EOIL is similar to that for the Chemical IodineOxygen Laser (COIL), except that dissociation of the reagent iodine, I₂,occurs through rapid reactions with atomic oxygen rather than the muchless efficient energy transfer from O₂(a). COIL systems use an aqueouschemical process to generate O₂(a), therefore no atomic oxygen ispresent, and I₂ is dissociated by a complex multi-step process whichconsumes a portion of the O₂(a).

SUMMARY OF THE INVENTION

The invention, in one embodiment, features an efficient technique togenerate energetic singlet oxygen metastables, O₂(a¹Δ) at high dischargepower and in a high-throughput gas flow using a microwave drivendischarge device. In one embodiment, the device is a Microwave DrivenJet (MIDJet) (e.g., as available from Physical Sciences Inc. in Andover,Mass.). The energetic singlet oxygen metastables can be used in EOILsystems.

EOIL's gas-phase electric discharge generation of the active oxygenspecies offers substantial improvements in efficiency and weightlimitations of atomic iodine laser systems. EOIL technology iscompatible with non-hazardous, liquid-free chemical requirements,on-board power generation, turn-key operation, substantial weightreduction, and closed-cycle systems. These properties are essential formany advanced Department of Defense applications, including groundbased, airborne and space deployment. A microwave driven dischargedevice (e.g., a MIDJet singlet oxygen generator) is an attractive andcomplementary alternative to other EOIL techniques currently beingpursued, e.g., rf discharge and pulser-sustainer methods, because of itsinherent capability for high input power, high electrical efficiency,and high mass flow rate, all of which can increase the output of EOILlaser devices. Furthermore, commercially available magnetrons can beused to reduce cost and increase reliability. Using a microwave drivendischarge device, an EOIL system can operate at a discharge power of 1kW or higher (e.g., in the 0.1-1 MW range).

In one aspect, the invention features a laser device including anoptical resonator, a microwave driven discharge device, and a source fora second gas. The microwave driven discharge device is disposed relativeto the optical resonator. The microwave driven discharge device operatesat a discharge power and gas flow rate to produce a selected amount ofenergetic singlet oxygen metastables flowing in the direction of theoptical resonator. The second source for the second gas is disposedbetween the optical resonator and the microwave driven discharge device.The second gas reacts with the selected amount of energetic singletoxygen metastables to form an excited species in an amount sufficient tosupport lasing of the excited species in the optical resonator.

In another aspect, the invention features a method for providing laseroutput. A flow of ground state oxygen and a substantially inert gas isdirected into a microwave cavity to produce energetic singlet oxygenmetastables. Discharge power and gas flow rate are optimized to producea selected amount of the energetic singlet oxygen metastables. A flow ofa third gas is directed to react with the selected amount of theenergetic singlet oxygen metastables to form an excited species in anamount sufficient to support lasing of the excited species in an opticalresonator disposed relative to the microwave cavity.

In another aspect, the invention features a laser device including meansfor directing a flow of ground state oxygen and a substantially inertgas into a microwave cavity to produce energetic singlet oxygenmetastables. The laser device includes means for optimizing dischargepower and gas flow rate to produce a selected amount of the energeticsinglet oxygen metastables. The laser device includes means fordirecting a flow of a third gas to react with the selected amount of theenergetic singlet oxygen metastables to form an excited species in anamount sufficient to support lasing of the excited species in an opticalresonator disposed relative to the microwave cavity.

In further examples, any of the aspects above, or the embodimentsdescribed herein, can include one or more of the following features.

In some embodiments, the microwave driven discharge device comprises amicrowave cavity, an injector directing a flow of ground state oxygenand a substantially inert gas to the microwave cavity, and a microwavepower source supplying the discharge power directly to the microwavecavity to form a plasma discharge flowing through an output nozzle ofthe microwave cavity. The plasma discharge can include the selectedamount of energetic singlet oxygen metastables.

The discharge power and the gas flow rate can be selected to increasethe selected amount of energetic singlet oxygen metastables whilecontrolling electron temperature in the microwave cavity.

The discharge power and the gas flow rate can be selected to increasethe selected amount of energetic singlet oxygen metastables whilecontrolling gas temperature in the microwave cavity.

The injector can direct a flow of NO with the ground state oxygen andthe substantially inert gas to the microwave cavity. The microwavecavity can be cooled with water.

The nozzle can be disposed between the optical resonator and the source.The nozzle can effect a supersonic expansion of the gas flow includingthe excited species into the optical resonator. The optical resonatorcan be disposed in a subsonic flow region of the laser device. The laserdevice can be an open system or a closed system.

The microwave driven discharge device can include or can be a MIDJetgenerator. In some embodiments, the microwave driven discharge deviceincludes a plurality of MIDJet generators disposed relative to theoptical resonator to increase the power of the laser output.

The microwave power source can deliver about 1 kW to about 100 kW. Thepressure in the microwave cavity can be less than 100 torr. The outputnozzle diameter can be less than 30 mm.

Using a MIDJet 1-5 kW system operating on flowing O₂/He gas mixtures at1-2 kW and reduced pressures (e.g., about 30-70 torr) in the cavity, anon-equilibrium discharge plasma can be produced with concomitant highyields of O₂(a) in the effluent flow. Orifice diameters can be less thanabout 15 mm when operating at a frequency of 2450 MHz. Orifice diameterscan be less than about 30 mm when operating at a frequency of 915 MHz.Additionally, O₂(a) yields can increase markedly with decreasing O₂ molefraction in the He diluents, as well as with the decreased dischargepressure.

The O₂(a) yields can exceed 15% for O₂ mole fractions less than 0.2, and36% for 2% O₂. For example, the O₂(a) molar flow rate can beapproximately 1 mmole/s at 20% O₂, although larger flow rates can beachieved depending on the application. A wall-plug efficiency of between10-30% can be achieved, although larger or smaller efficiencies can beachieved depending on the application. The power available for lasingcan be in the 30-50 W range, although higher and lower laser powers canbe achieved, for these flow conditions. The temperature in thesupersonic laser cavity can be between about 100-250 K. Thus,MIDJet-generated O₂(a) yields and flow rates are capable of producingatomic iodine population inversions and lasing with a properly designedsupersonic reacting flow system at discharge powers up to 1 MW.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention described above, together with furtheradvantages, can be better understood by referring to the followingdescription taken in conjunction with the accompanying drawings. Thedrawings are not necessarily to scale, emphasis instead generally beingplaced upon illustrating the principles of the invention.

FIG. 1 is a cross sectional view of one embodiment of a microwave drivendischarge device.

FIG. 2 is a cross sectional view of one embodiment of an EOIL, includingmicrowave driven discharge device for providing singlet oxygenmetastables.

FIG. 3 is a schematic illustration of one embodiment of chemistry of anEOIL, including microwave driven discharge device for providing singletoxygen metastables.

FIG. 4 is a diagram of one embodiment of a supersonic, high-power EOILdriven by multiple microwave driven discharge devices.

FIG. 5 is a cross sectional view of one embodiment of an EOIL, includingmicrowave driven discharge device for providing singlet oxygenmetastables.

FIG. 6 is a graph of cross sections required for electron-impactexcitation of O₂(a) excitation, O₂ dissociation, and O₂ ionization.

FIG. 7 is a graph of computed electron energy distributions functionsfor a discharge-excited O₂/Ar mixture having 10% O₂ in Ar.

FIG. 8 is a graph of computed electron energy distributions functionsfor a discharge-excited O₂/Ar mixture with varying O₂ mole fractionswhen E/N is equal to 50 Td.

FIG. 9 is a graph of an overlap of electron energy distributions thatshows electron impact excitation cross sections for O₂(a) excitation, O₂dissociation, and O₂ ionization, when E/N is equal to 10 Td and 100 Tdin 10% O₂/Ar.

FIG. 10 is a graph of the effect of variations in O₂ mole fraction oncomputed electron-impact rate coefficients for O₂(a) excitation, O₂, andtotal ionization of discharge-excited O₂/Ar mixtures when E/N is equalto 50 Td.

FIG. 11 is a graph of O₂(a) emission spectra with a 1 kW dischargepower, various O₂ mole fractions, and various plenum pressures.

FIG. 12 is a graph of I* emission spectra with a 1 kW discharge power,various O₂ mole fractions, various plenum pressures, and an I₂ flow rateequal to 0.18 mole/s.

FIG. 13 is a graph of atomic iodine absorption with a 1 kW dischargepower, various O₂ mole fractions, and various plenum pressures.

FIG. 14 is a graph of gain measurement at 1 kW discharge power.

FIG. 15 is a graph of O₂(a) yield generated by the MIDJet discharge witha 1 kW power and various plenum pressures.

FIG. 16 is a graph of O₂(a) flow rates for the MIDJet singlet oxygenmetastable driven EOIL driven with a 1 kW.

FIG. 17 is a graph of O₂(a) power available in the flow for selectedflow temperatures, corresponding to observed MIDJet O₂(a) yields at 1 kWdischarge power and 45-50 torr microwave driven discharge devicepressure.

DESCRIPTION OF THE INVENTION

FIG. 1 is a cross sectional view of one embodiment of a microwave drivendischarge device 100 for providing energetic molecular species. Incertain embodiments, the microwave driven discharge device 100 can be aMIDJet singlet oxygen metastable generator. An exemplary microwavedriven discharge device is described in U.S. Pat. No. 5,793,013 to Readet al., the entire disclosure of which is herein incorporated byreference in its entirety. The microwave driven discharge device 100includes an input waveguide 110, a microwave launcher 120, and amicrowave cavity 130. The microwave cavity 130 includes inner microwavecavity conductor portion 112, a gas inlet nozzle 140, an output nozzle170, a housing 180, and a microwave passing window 190.

The microwave cavity 130 confines plasma 194 without the use of adischarge tube. The input waveguide 110 of the microwave cavity 130 ismovable along a longitudinal axis 164, allowing for the length of themicrowave cavity 130 to be adjusted. Adjusting the length of themicrowave cavity 130 achieves resonance in a particular mode ofoperation, such as a TM₀₁ mode. The TM₀₁ mode has an axial electricfield maximum at the ends of the microwave cavity 130 that is desirablefor concentrating power near the output nozzle 170.

In some embodiments, the housing 180 is actively cooled (e.g., withwater). In one embodiment, the housing 180 can be brass and the interiorsurfaces forming the microwave cavity 130 can be gold-flashed brass,although other metallic materials can be used.

The gas inlet nozzle 140 can be used to introduce a gas suitable forionization into the microwave cavity 130 and for creating a velocity andswirl adequate to stabilize a discharge plasma in all orientationswithin the microwave cavity 130. More than one gas inlet nozzle 140 canbe used. The gas inlet nozzle 140 is preferably disposed at an angle ofabout 25 degrees to 70 degrees with respect to the longitudinal axis 164of the microwave cavity 130. The angle of orientation of the gas inletnozzle 140 along with the velocity at which the gas is introduced andthe pressure within the microwave cavity 130 control the vorticity ofthe gas within the microwave cavity 130. A specific vorticity within themicrowave cavity 130 can be chosen to compensate for centripetal forcesexperienced by the gas. In one embodiment, the gas inlet nozzle 140 cantake the form of a converging or diverging nozzle. A converging nozzlecan increase the velocity of the gas and cause impingement of the gasagainst the interior surfaces of the microwave cavity 130.

In some embodiments, the output nozzle 170 can have a profilecorresponding to either a conical, a quasi-parabolic, a cylindrical, ora parabolic taper. In some embodiments, the output nozzle 170 is made ofeither metal, ceramic, graphite, or a mixture thereof to resist erosionfrom the materials utilized in spraying. In one embodiment, the outputnozzle 170 can have an aperture with a diameter in the range of about0.5 mm to 50 mm. In an embodiment where the microwave driven dischargedevice operates at 2450 MHz, the aperture diameter can be in the rangeof about 1 mm to 15 mm (e.g., about 10 mm). In an embodiment where themicrowave driven discharge device operates at 915 MHz, the aperturediameter can be in the range of about 15 mm to 30 mm (e.g., about 20mm). In some embodiments, the output nozzle 170 can have a variableaperture for controlling output gas velocity or cavity pressure.

The microwave passing window 190 can be formed of a materialsubstantially transparent to microwave radiation. The microwave passingwindow 190 is a pressure plate for maintaining a certain pressure in themicrowave cavity 130. The microwave passing window 190 can be of varyingthickness. In one embodiment, the microwave passing window 190 can be ofa thickness in a range of 6 mm to 12 mm, e.g., a thickness that iscrack-resistant to pressures in a range of 0 psig to 150 psig.

The microwave launcher 120 includes an input waveguide 110. The inputwaveguide 110 can be a coaxial launcher with an inner microwave cavityconductor portion 112 and an outer microwave cavity conductor portion114. An end of the inner microwave cavity conductor portion 112 extendsthrough the microwave passing window 190. The microwave cavity 130 cansupport a transverse electromagnetic mode to generate a more efficientand uniform spray. In one embodiment, the input waveguide 110 candischarge at frequency 2450 MHz. In one embodiment, the input waveguide110 can discharge at frequency 915 MHz. In one embodiment, the inputwaveguide 110 can discharge at a power of 40-120 W, the discharge powerthat is suitable for room temperature discharge-flow experiments.

In some embodiments, the discharge power of the input waveguide 110 andthe gas flow rate in the microwave cavity 130 can be controlled so thata selected amount of energetic singlet oxygen metastables is generated.Increasing the discharge power can create an undesirable increase intemperature of the electrons and/or gas inside the microwave cavity 130.The gas flow rate can be adjusted to control the temperature increase inthe electrons and/or gas. For example, to increase the amount ofenergetic singlet oxygen metastables, the discharge power and/or gasflow rate can be increased.

FIG. 2 is a cross sectional view of one embodiment of an apparatus 200including an EOIL and a microwave driven discharge device for providingsinglet oxygen metastables. The apparatus 200 includes microwave drivendischarge device region 210, a subsonic flow section 220, and asupersonic flow section 230.

The microwave driven discharge device region 210 includes a microwavedriven discharge device 100, an input waveguide 110, and a microwavepower supply 205. The microwave driven discharge device 100 includes afirst gas inlet nozzle 140 and a second gas inlet nozzle 212. Themicrowave power supply can be a magnetron, which provides power to theinput waveguide 110. In one embodiment, the microwave power supply 205can be a commercially available magnetron. Magnetrons are electricallyefficient and available for high power operation in the range of 1 kW to100 kW.

The input waveguide 110 supplies an electrical microwave discharge to aflowing oxygen and inert gas mixture that is injected into the microwavecavity 130 through the gas inlet nozzles 140 and 212. The electricalmicrowave discharge to the flowing and inert gas mixture generatessinglet oxygen metastables and atomic oxygen in the plasma 194. Thesinglet oxygen metastables and atomic oxygen are received by thesubsonic flow section 220. The subsonic flow section 220 includes asubsonic flow cavity 222, three gas inlet nozzles 224, 226 and 228, anda nozzle 232. The gas inlet nozzles 224 can be used to introduce abuffer gas, such as NO₂. The gas inlet nozzles 226 and 228 injectmolecular iodine into the subsonic flow section 220. In someembodiments, the subsonic flow cavity 222 has a short section, allowingfor a 1 inch axisymmetric flow of the singlet oxygen metastables andatomic iodine. The short section can merge to a water cooled transitionsection that transforms the flow into a rectangular duct approximately 1cm×5 cm. The nozzle 232 can be 1.5 mm to produce Mach 2.6 flow.

The supersonic flow section 230 receives expanding gas from the nozzle232 of the subsonic flow section 220. The supersonic flow section 230includes an optical resonator region 240 including a pair of windows245. In some embodiments, the windows 245 can be the end mirrors of theoptical resonator. The supersonic flow section 230 includes a flow tube250 in communication with a pump (e.g., an open system). In someembodiments, the apparatus 200 is pumped through a high-conductance gatevalve and foreline by a 2150 cfm(air) blower and forepump combination.

In some embodiments, the flow in the supersonic flow section 230 candiverge with a half-angle of 2 or 4 degrees. The angle of divergence canbe sufficient to offset boundary layer growth. Additionally, thedistance from the nozzle 232 to the end of the supersonic flow section230 can be 12 cm. In some embodiments, the supersonic flow section 230is aluminum and internally coated with Teflon to mitigate O₂(a) walllosses.

The mechanism for the reactions between molecular iodine and the singletoxygen metastables and atomic oxygen is:

$\begin{matrix}\begin{matrix}\left. {O + I_{2}}\rightarrow{{IO} + I} \right. & {\mspace{95mu}{I_{2}\mspace{14mu}{dissociation}\mspace{14mu}{part}\mspace{14mu} 1}}\end{matrix} & {{EQN}.\mspace{14mu} 1} \\{\begin{matrix}\left. {O + {IO}}\rightarrow{O_{2} + I} \right. & {\mspace{79mu}{I_{2}\mspace{14mu}{dissociation}\mspace{14mu}{part}\mspace{14mu} 2}}\end{matrix}\mspace{14mu}} & {{EQN}.\mspace{14mu} 2} \\{\begin{matrix}\left. {{O_{2}(a)} + I}\leftrightarrow{{O_{2}(X)} + I^{*}} \right. & {\;{I^{*}\mspace{14mu}{excitation}}}\end{matrix}\mspace{14mu}} & {{EQN}.\mspace{14mu} 3} \\{\begin{matrix}\left. {O + I^{*}}\rightarrow{O + I} \right. & {\mspace{101mu}{I^{*}\mspace{14mu}{quenching}}}\end{matrix}\mspace{11mu}} & {{EQN}.\mspace{14mu} 4}\end{matrix}$

The two dissociation reactions shown by EQN. 1 and EQN. 2 have neargas-kinetic rate coefficients, and rapidly react in less time then thetime it takes for reagent mixing to produce complete dissociation if[O]>2[I₂]. In some embodiments, I can be generated by predissociatingI₂. In some embodiments, I can be formed from a iodine containingcompound.

The I* excitation by the energy transfer from O₂(a) is near-resonant andreversible, as shown in EQN 3. The forward (k_(f)) and reverse (k_(r))rate coefficients stand in the ratio of a temperature-dependentequilibrium constant:k _(f) /k _(r) =K _(eq)(T)=0.75exp(402/T)  EQN. 5

Quenching of I* by O, shown by EQN. 4, can result in a significant lossof I*, and thereby in O₂(a), for typical discharge-generated Oconcentrations. Titrations with NO₂ and/or NO can ensure that [O] islarge enough to dissociate I₂, but small enough to minimize I*quenching. The gas inlet nozzle 224 can inject nitrogen dioxide, NO₂,into the subsonic flow section 220, to control the O concentrationthrough titrations with the NO₂. In one embodiment, the gas inlet nozzle224 can inject NO, into the subsonic flow section 220 to minimize the I*quenching. In some embodiments, NO can be introduced to the microwavedischarge along with oxygen and an inert gas.O+NO+M→NO₂+M  EQN. 6O+NO₂→NO+O₂  EQN. 7

The I* excitation mechanism results in a rate law of the form:d[I*]/dt=k _(f)[O₂(a)][I]−k _(r) [I*][O ₂(X)]−k _(O) [I*][O]  EQN. 8where the net effect of I* quenching is to convert O₂(a) to O₂(X) as thereaction time increases. For slow quenching, a quasi-steady stateapproximation for [I*] gives:[I*]/[I]≅k _(f)[O₂(a)]/{k _(r)[O₂(X)]+k _(O)[O]}  EQN. 9As k_(O)[O] decreases through reduction of [O], k_(O)[O]<k_(r)[O₂(X)],the expression approaches a true steady-state relationship given by:[I*]/[I]≅k _(f)[O₂(a)]/k _(r)[O₂(X)]=K _(eq)(T)[O₂(a)]/[O₂(X)]  EQN. 10EQN. 10 defines the maximum [I*]/[I] ratio that can be achieved for agiven [O₂(a)]/[O₂(X)] and temperature. Through consideration of atomiciodine state dynamics and degeneracies, it can be shown that populationinversion and positive gain are achieved if [I*]/[I] is greater than0.5. The apparatus 200 can be driven with an input power of 0.07 kW to0.1 kW such that a positive I*→I gain in the subsonic flow section 220for a temperature of 350 K as shown in “Observations of Gain on theI(2P1/2→2P3/2) Transition by Energy Transfer from O2(a1Δg) Generated bya Microwave Discharge in a Subsonic Flow Reactor,” by W. T. Rawlins, S.Lee, W. J. Kessler, and S. J. Davis, Appl. Phys. Lett. 86, 051105(2005), which is incorporated herein by reference in its entirety. EQN.10 can be used to define a minimum [O₂(a)]/[O₂(X)] ratio required toachieve gain.

The total O₂, [O₂]o, introduced into the apparatus 200, is given by:[O₂]_(o)=[O₂(X)]+[O₂(a)]+[O]/2  EQN. 11

The ratio [O₂(a)]/[O₂]₀ is defined as the yield of O₂(a), Y_(Δ),produced by the discharge and is a fundamental metric for theperformance of the apparatus 200. The combination of EQN. 10 and EQN. 11defines a minimum O₂(a) yield, Y_(Δ), that must be exceeded in order toproduce a positive gain, in the limit of negligible [O]. For asupersonically cooled flow of approximately 200 K, a yield of O₂(a),Y_(Δ), greater than 8.2% is required to achieve gain. The apparatus 200supplied with an input power in the 0.1 kW to 1.0 kW range exhibitsO₂(a) yields, Y_(Δ), in the 20-35% range for dilute O₂/rare-gasmixtures, well above the required threshold even at room temperature.

The total O₂(a) power produced by the EOIL discharge is the product ofthe O₂(a) molar flow rate and energy shown by:P_(Δ)=F_(O) ₂ Y_(Δ)E_(Δ)  EQN. 12where F_(O) ₂ is the molar flow rate of O₂, Y_(Δ) is the O₂(a) yield,and E_(Δ) is O₂(a) energy, 94.369 kJ/mole. An alternative metric, whichrequires specification of the laser cavity temperature, is the O₂(a)power available above the gain threshold:P _(Δ), avail=F _(O) ₂ (Y _(□) −Y _(o)(T)) E _(Δ) =P _(Δ)(1−Y _(o)(T)/Y_(Δ))  EQN. 13where Y_(o)(T) is the threshold O₂(a) yield as given by EQN. 10 and 11.The available power is the theoretical maximum that can be extracted aslaser power if there are no losses in the system.

Typically, Y_(o)(T) is greatly reduced by supersonic expansion in thesupersonic flow section 230 so the available power for laser output fromthe optical resonator 240 increases with decreasing cavity temperature.Limitations of reagent mixing, reaction kinetics, and optical losses canreduce the power extracted in a practical system. The total andavailable O₂(a) power can be important figures of merit in theevaluation of the performance of the apparatus 200. For example, theperformance can scale as the product of the O₂(a) yield and the totaloxygen flow rate. Both high O₂(a) yields and high oxygen flow rates canbe used to scale laser powers into the kW range and higher. For example,if Y_(Δ) is approximately equal to 25% and Y_(o)(200 K) is approximatelyequal to 8%, then P_(Δ), avail is approximately equal to (⅔) P_(Δ). Anavailable laser power of 1 kW can require a total O₂(a) power of 1.5 kWand an oxygen flow rate, in rare gas diluents, of approximately 0.06mole/s (80 l/min at STP).

FIG. 3 shows chemistry of an EOIL apparatus. In some embodiments, NO isinjected into the microwave driven discharge device 100 with oxygen andan inert gas. Helium is shown in FIG. 3, although other gases can beused. Adding NO can reduce the O+I* quenching effect and improveefficiency of the discharge. The NO flow rate can be optimized toproduce maximum gain for a given I* flow rate. For example, the NO flowrate can be decreased with increasing I₂ flow rate. The NO flow rate canbe in the range of about 0.4 to 0.5 mmole/s.

An optical resonator is positioned relative to the supersonic flowsection 230. The optical resonator can include, for example, two mirrors250. The mirrors can be 1 inch diameter and have a reflectivity of99.9997%. Each mirror can be mounted on a three-point tilt control andset back approximately 6.5 inches from the side edge of the twodimensional supersonic flow field, on opposite sides of the flow. Themirrors can be centered approximately 4.35 cm downstream from thenozzle. In some embodiments, the I₂ can be injected into the subsonicflow section 220. The subsonic flow section 220 can be about 1 cm, 4 cmor 8 cm from the microwave driven discharge device section 210.

For a 5% O₂/He mixture at 47 mmole/s, NO approximately equal to 0.4mmole/s through the microwave driven discharge device 100 at 33 torr, 24torr in the subsonic flow section 220, 1 kW of discharge power, and I₂heated in the range of 308 to 318 degrees Kelvin, laser output can beapproximately 20 mW. The total singlet oxygen power in the flow can beabout 41 W. For a discharge flow rate of 82 mmole/s and 70 torr in themicrowave driven discharge device section 210, laser output can beapproximately 110 mW.

FIG. 4 shows an apparatus 400 including multiple microwave drivensinglet oxygen metastable generators, 410 a, 410 b, . . . , 410 n,generally generator 410. Each generator 410 includes, respectively, awaveguide, 420(a), 420(b), . . . 420(n), generally waveguide 420. Eachgenerator 410 produces a singlet oxygen flow rate that is additive inthe subsonic flow section 220. An increase in singlet oxygen can providean increase in I* in the supersonic flow section 230, resulting in anincrease in laser output power. Each generator 410 can be a MIDJet. Theoptical resonator includes windows 445 in communication with the flowtube, and mirrors 450 defining the ends of the optical resonator cavity.

FIG. 5 is a cross sectional view of one embodiment of an apparatus 500including an EOIL and a microwave driven discharge device 100 forproviding singlet oxygen metastables. The optical resonator, includingmirrors 450, is disposed in the subsonic flow region 220 of theapparatus 500. Operating in the subsonic flow section 220 allows foroperation at higher temperature than the supersonic region andcorrespondingly higher O₂(a) concentrations.

The apparatus 500 can include an open pumping system or a closed pumpingsystem. For example, in a closed system, apparatus 500 can include a gasregeneration system for recycling gas. Gas from the flow tube 510 can berecycled and re-introduced into the microwave driven discharge device100 and/or the subsonic flow region 220 upstream from the opticalresonator.

The gas regeneration system can include an outlet tube 520 from flowtube 510, a cold trap 530, a turbo pump 540, and a heat exchanger 550.Effluent gas from flow tube 510 can pass through one or more of coldtrap 530, turbo pump 540, and heat exchanger 550. Cold trap 530 can trapiodine from the effluent gas. The trapped iodine can be reintroducedinto inlet tube 226. Turbo pump 540 can raise the pressure of a gasbeing fed into the microwave driven discharge device 100. For example,oxygen, NO or the substantially inert gas can be reintroduced into gasinlet nozzle 140 or 212. Heat exchanger 550 can control the temperatureof the recycled gas. For example, if the gas being reintroduced into themicrowave driven discharge device 100 becomes heated beyond apredetermined temperature, the heat exchanger 550 can cool the gasbefore reintroducing the gas into gas inlet nozzle 140 or 212.

In some embodiments, each MIDJet singlet oxygen metastable generatorscan be supplied with 100 kW of power to drive an EOIL in the 10 kW classor greater. The ultimate efficiency at elevated power depends on acomplex interplay of several factors such as the power to flow rateratio, trade-offs between O₂(a) yield and oxygen flow rate, dischargeionization rate, discharge plenum temperature and pressure, supersonicexpansion characteristics, O-atom effects, I₂ injection and mixingdynamics, and optical power extraction requirements. In addition,commercially available microwave power systems that operate at 30 kW to100 kW and low frequencies, e.g., 915 MHz at 30 kW, to enable a largedischarge volume, thus a larger diameter for the supersonic flow section230 and a reduction in the power to flow rate ratio for a given plenumpressure.

O₂(a) can be generated in a variety of electric discharges andconfigurations. In some embodiments, the microwave driven dischargedevice can be an electrodeless microwave discharge at 2450 MHz. In someembodiments, the microwave driven discharge device includes a microwavecavity that is an Evenson-type resonant cavity. The discharges can be ata power of about 40 to 120 W, and an O₂/Ar or O₂/He mixture can have apressure of a few torr. The E/N range can be about 50 to 100 Td. In someembodiments, for high power and high throughput operation, the microwavecavity can have a pressure of 40 to 50 torr, the input waveguide can bea coaxial wire providing an input power of 1 kW, and an E/N of about 30to 40 Td, high power and high throughput.

FIG. 6 is a graph of electron energy distribution cross sectionsrequired for electron-impact excitation of O₂ to form O₂(a) excitation,O₂ dissociation and O₂ ionization. The rate coefficient for eachelectron-impact excitation process can modeled by the convolutionintegral of the energy-dependent excitation cross sections σ(E) and theelectron energy distribution N(E):

$\begin{matrix}{k = {\left( \frac{2\; e}{m} \right)^{1/2}{\int_{0}^{\infty}{{\sigma(E)}{N(E)}E\ {\mathbb{d}E}}}}} & {{EQN}.\mspace{14mu} 14}\end{matrix}$Cross sections for O₂(a) excitation, O₂ dissociation, and O₂ ionizationare shown in FIG. 4. Although O₂(a) lies at an energy of approximately 1eV, O₂(a) excitation cross section peaks at 6 to 7 eV. In addition,electron energies greater than 12 eV are required to maintain ionizationin the plasma. Thus, the electron energy distribution or “temperature”can be “hot” enough to provide sufficient overlap with these crosssections. An electron energy distribution which gives significant O₂(a)excitation and ionization can also give substantial O-atom production.

FIG. 7 is a graph of computed electron energy distributions functionsfor a discharge-excited O₂/Ar mixture having 10% O₂ in Ar. Two controlson the electron energy distribution are the E/N of the discharge and therelative amounts of O₂ and rare gas (e.g., He or Ar) in the gas mixture.E/N is the ratio of the field strength E, governed by the applied powerand discharge geometry, to the total number density N, governed bypressure and temperature. With an increasing E/N, the fraction ofhigh-energy electrons increases, signifying increasing electron“temperature.”

FIG. 8 is a graph of computed electron energy distributions functionsfor a discharge-excited O₂/Ar mixture with varying O₂ mole fractionswhen E/N is equal to 50 Td. With a decreasing O₂, the fraction ofhigh-energy electrons increases, signifying increasing electron“temperature.” Increases in the high-energy component of the electronenergy distribution result in larger overlap integrals that have keyelectronic excitation cross sections.

FIG. 9 is a graph of the overlap of electron energy distributions thatshows electron impact excitation cross sections for O₂(a) excitation, O₂dissociation, and O₂ ionization, when E/N is equal to 10 Td and 100 Tdin 10% O₂/Ar. The 10 Td distribution provides power-efficient O₂(a)excitation, in that very little power is expended on O₂ dissociation.However, the poor overlap with the ionization cross section results in avery low ionization rate and consequently low electron number density.The 100 Td distribution gives greater overlap with both the O₂(a)excitation cross section and the O₂ ionization cross section, but at theexpense of increased O₂ dissociation. The O₂(a) excitation rate is givenby the product k_(exc)[e⁻][O₂]. Both k_(exc) and [e⁻], and hence theyield of O₂(a), can be considerably enhanced through use of larger E/Nand/or lower O₂ mole fraction to achieve more energetic electron energydistributions. This illustrates the basic conundrum of O₂(a) generationfor EOIL: high power utilization efficiency is optimized by “cold”electron energy distributions (e.g. lower E/N), however high O₂(a)production rate and therefore high O₂(a) yield require more energeticdistributions (e.g. higher E/N).

FIG. 10 is a graph of the effect of variations in O₂ mole fraction oncomputed electron-impact rate coefficients for O₂(a) excitation, O₂disassociation, and total ionization of discharge-excited O₂/Ar mixtureswhen E/N is equal to 50 Td. The excitation rate coefficient increasesmodestly with decreasing O₂ fraction. However, the O₂ ionization ratecoefficient increases by almost two orders of magnitude from 80% O₂ to5% O₂, signifying a large increase in the ion pair production rate andin the electron number density. The Ar ionization rate coefficient,which has a higher energy threshold, is even more sensitive to the O₂fraction, and becomes an important contributor for the dilute mixtures.

An approximation to the zeroth order for the O₂(a) production is:[O₂(a)]≈k _(exc) [e ⁻][O₂]τ  EQN. 15where τ denotes the gas residence time in the discharge. The gasresidence time for the low-power Evenson-type resonant cavity embodimentin the discharge is approximately 0.2 ms. The gas residence timecorresponds to an effective O₂(a) “loss” rate that is faster thancollision losses within the discharge for anticipated electron numberdensities. The O₂(a) yield is then simply approximately k_(exc)[e⁻]τ,and is thus dependent on the electron energy distribution via E/N and O₂fraction. The gas residence time in the 1 kW coaxial discharge can belonger (e.g., approximately 4.0 ms).

In one embodiment, singlet oxygen metastables can be generated at highpower (e.g., 1 kW to 2 kW) and high throughput. Gas mixtures of O₂/Heand O₂/I₂/He can be injected into the plenum via a tangential jet togive a swirl flow confined near the axis of the cavity. The dischargeeffluent can expand at about Mach 2 through the nozzle into thesupersonic flow region. The nozzle can be a water-cooled boron nitridenozzle. The I* produced can be sufficient to produce lasing in theoptical resonator. The O₂/He mixture can be about 2-50% O₂ in He, theplenum pressures can be about 35-50 torr, the total discharge gas flowrate can be about 35 to 42 mmole/s, the discharge power can be about 1kW, and the discharge temperature can be about 1000 K. In someembodiments, more dilute O₂/He mixtures can be used with a 400 cfm (air)pumping speed that can limit the supersonic expansion conditions toM=1.8, T=500 K, and P=7.5 torr. The limited supersonic expansionconditions can be due to relatively low pumping capacity for helium. Insome embodiments, a MIDJet singlet oxygen metastable driven EOIL iscapable of operation up to 5 kW discharge power, with proportionatelyhigher gas flow rates to optimize O₂(a) production. In one embodiment,the MIDJet singlet oxygen metastable driven EOIL can operate with a 1 kWdischarge power and an E/N approximately equal to 30-43 Td. For alow-power subsonic reactor with a supply power of 70-100W and amicrowave cavity pressure of 1-3 torr, the yield of O₂(a) can increaseas the mole fraction of O₂ decreases. For example, with a ˜5% O₂/Armixture, 20-25% O₂(a) yields can be obtained. These yields can producepositive I*-I gain at room temperature. A positive gain at 350 K can beachieved when NO₂ is used to suppress atomic oxygen effects.

In one embodiment, a MIDJet singlet oxygen metastable driven EOIL wasdriven with a 1 kW input power. The O₂(a) yield results were similar tothose for the low-power subsonic reactor discussed above. The O₂(a) andI* emission spectra from the supersonic flow for different dischargeconditions are shown in FIGS. 11 and 12. As the O₂ mole fraction wasdecreased by a factor of 4, the O₂(a) emission intensity decreased byless than a factor of 2, signifying an increased O₂(a) yield. Inaddition, the I* emission intensity, proportional to [I*], was higherfor the lower O₂ mole fractions, signifying more I* excitation due to alarger O₂(a) yield. Small-signal gain measurements taken on the I*to Itransition by a tunable diode laser diagnostic are shown in FIG. 13. Thedata shows net absorption occurred at approximately 500 K. The flowexpansion and temperature are limited by low pumping speed for He,however the absorption decreased as the O₂(a) yield increased. Thissignified transfer of population from I to I*, commensurate with thebehavior of the I* emission.

FIG. 14 shows gain measurement at 1 kW discharge power. The conditionsfor these measurements were: 5% O₂/He at 47 mmole/s with ˜0.4 mmole/s NOadded through the discharge, 33 torr in the MIDJet discharge, and 24torr in the 2-D subsonic flow. The supersonic flow was probed by a diodelaser 4.35 cm downstream of the nozzle throat. The diode lasertransmission scans are shown for NO off (absorption) and on (gain), foran iodine source temperature of 37 C (˜5 μmole/s, [I2]o˜7×1013 cm-3 inthe subsonic flow). The scans have been corrected for etalon baselines,and are fit to Gaussian curves representing Doppler broadening. The linewidths correspond to temperatures of ˜135 K. Measurements furtherupstream, ˜2 cm downstream of the throat, showed slightly smallerpositive gain. 16% O₂ in He also showed positive gain. The iodine sourcetemperature can be increased to attain I₂ flow rates up to 33 μmole/sand peak gains up to 0.025%/cm, for a total flow rate of 82 mmole/s and70 torr in the discharge. Without NO being introduced into the dischargesmall positive gains are expected (few×10-3%/cm) for [I2]o less thanabout 1×10¹⁴ cm⁻³, and increasing absorption above that level. Theaddition of NO through the discharge reduces the O+I* quenching effect,as well as the efficacy of I* loss processes, resulting in positivegain.

The NO flow rate can be optimized to produce maximum gain for a given I2flow rate. Addition of excessive NO can cause O removal, such that O isnot in sufficient excess over I₂ to give adequate I₂ dissociation. TheNO flow rate can decrease with increasing I₂ flow rate: at higher [I₂]o,the O concentration at the I₂ inlet must be higher, and the O+I*quenching loss is more severe because of both the higher [O] and theincreasing O₂(a) loss with increasing [I]. In some embodiments, the gaincan be optimized at about 0.4 to 0.5 mmole/s of NO, and can decreases athigher NO flow rates.

FIG. 15 is a graph of O₂(a) yield generated by the MIDJet dischargeplenum with a 1 kW power source and various plenum pressures. Throughintegration of the O₂(a) emission spectra and application of theEinstein coefficient for the (a→X) transition, [O₂(a)] and O₂(a) yieldswere determined. The observed O₂(a) concentrations ranged from (0.7 to3)×10¹⁵ molecules/cm³ for total O₂ concentrations of (0.3 to 5)×10¹⁶molecules/cm³ in the 7.5 torr flow. These yields exceed 20% for O₂ molefractions below 10%.

FIG. 16 is a graph of O₂(a) flow rates for the MIDJet singlet oxygenmetastable driven EOIL driven with a 1 kW. The total O₂(a) power in theflow is indicated on the right hand axis. The O₂(a) flow rate peaked outat approximately 1 mmole/s, corresponding to a peak total O₂(a) power ofapproximately 100 W and a peak power efficiency of approximately 10% forthe generation of O₂(a), at an O₂ mole fraction of 20%. Using EQN. 13and a curve fit, the maximum power available for lasing was found, formultiple temperatures in the cavity as produced by supersonic expansion,180K, 200K and 250K with 45-50 torr in the MIDJet discharge plenum. Theresults are shown in FIG. 17. The available 02(a) powers above the gainthreshold were about 10 to 50 W with a maxima at O₂ mole fractions of10-15%. The observed O₂(a) yield is comparable to those used todemonstrate positive gain near room temperature and is larger than thoseused in previous EOIL laser demonstrations in supersonic flow.Furthermore, tens of watts of laser power can be extracted from this 1kW supply power discharge system with sufficient pumping capacity toadequately cool the flow, e.g., a pumping speed of at least 2000 cfm(air).

For discharge operation at 1 kW and higher powers, heat deposition andthe temperature of the plenum gas can be significant issues. Physicsdictates that the temperature rise due to heat deposition from theapplied discharge power scales as the ratio of the power to the molar(or mass) flow rate of the gas. If there were no active cooling of theMIDJet plenum, the temperature rise would be ˜1275 K for 1 kW power.However, for the water-cooled nozzle system, ΔT was approximately 700 K.Nevertheless, this results in a plenum gas temperature of 1000 K, whichnecessitates a Mach number M>3 to reach T<250 K in the supersonicexpansion. Therefore, a high-power EOIL system can be implemented withactive cooling of the plenum gas, preferably to temperatures below ˜800K so that Mach 2-3 nozzles can be used.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the technology asdefined by the appended claims.

1. A laser device comprising: an optical resonator; a plurality ofMIDJet generators disposed relative to the optical resonator, eachMIDJet generator operating at a discharge power and gas flow rate toproduce a selected amount of energetic singlet oxygen metastablesflowing in the direction of the optical resonator; and a source for asecond gas disposed between the optical resonator and the plurality ofMIDJet generators, the second gas reacting with the selected amount ofenergetic singlet oxygen metastables to form an excited species in anamount sufficient to support lasing of the excited species in theoptical resonator.
 2. The laser device of claim 1 wherein each MIDJetgenerator comprises: a microwave cavity; an injector directing a flow ofground state oxygen and a substantially inert gas to the microwavecavity; and a microwave power source supplying the discharge powerdirectly to the microwave cavity to form a plasma discharge flowingthrough an output nozzle of the microwave cavity, the plasma dischargeincluding the selected amount of energetic singlet oxygen metastables.3. The laser device of claim 2 wherein the discharge power and the gasflow rate are selected to increase the selected amount of energeticsinglet oxygen metastables while controlling electron temperature in themicrowave cavity.
 4. The laser device of claim 2 wherein the dischargepower and the gas flow rate are selected to increase the selected amountof energetic singlet oxygen metastables while controlling gastemperature in the microwave cavity.
 5. The laser device of claim 2wherein the injector directs a flow of NO with the ground state oxygenand the substantially inert gas to the microwave cavity.
 6. The laserdevice of claim 2 wherein the microwave cavity is cooled with water. 7.The laser device of claim 1 further comprising a nozzle disposed betweenthe optical resonator and the source, the nozzle effecting a supersonicexpansion of the gas flow including the excited species into the opticalresonator.
 8. The laser device of claim 1 wherein the optical resonatoris disposed in a subsonic flow region of the laser device.
 9. The laserdevice of claim 2 wherein the microwave power source delivers about 1 kWto about 100 kW.
 10. The laser device of claim 2 wherein pressure in themicrowave cavity is less than 100 torr.
 11. The laser device of claim 2wherein the output nozzle diameter is less than 30 mm.
 12. A method forproviding laser output, comprising: directing into each of a pluralityof MIDJet generators a flow of ground state oxygen and a substantiallyinert gas to produce energetic singlet oxygen metastables; optimizingdischarge power and gas flow rate of each of the plurality of MIDJetgenerators to produce a selected amount of the energetic singlet oxygenmetastables; and directing a flow of a third gas to react with theselected amount of the energetic singlet oxygen metastables to form anexcited species in an amount sufficient to support lasing of the excitedspecies in an optical resonator disposed relative to the plurality ofMIDJet generators.
 13. The method of claim 12 further comprisingcontrolling the discharge power and the gas flow rate to increase theselected amount of energetic singlet oxygen metastables whilecontrolling electron temperature in each of the plurality of MIDJetgenerators.
 14. The method of claim 12 further comprising controllingthe discharge power and the gas flow rate to increase the selectedamount of energetic singlet oxygen metastables while controlling gastemperature in each of the plurality of MIDJet generators.
 15. Themethod of claim 12 further comprising injecting a flow of NO with theground state oxygen and the substantially inert gas to each of theplurality of MIDJet generators.
 16. The method of claim 12 furthercomprising disposing a nozzle between the optical resonator and the flowof the third gas, the nozzle effecting a supersonic expansion of the gasflow including the excited species into the optical resonator.
 17. Themethod of claim 12 further comprising disposing the optical resonator ina subsonic flow region of the laser device.
 18. The method of claim 12further comprising cooling each of the plurality of MIDJet generatorswith water.
 19. The method of claim 12 further comprising operating at adischarge power of about 1 kW to about 100 kW.
 20. A system forproviding laser output, comprising: a plurality of MIDJet generators;means for directing into each of the plurality of MIDJet generators aflow of ground state oxygen and a substantially inert gas to produceenergetic singlet oxygen metastables; means for optimizing dischargepower and gas flow rate of each of the plurality of MIDJet generators toproduce a selected amount of the energetic singlet oxygen metastables;and means for directing a flow of a third gas to react with the selectedamount of the energetic singlet oxygen metastables to form an excitedspecies in an amount sufficient to support lasing of the excited speciesin an optical resonator disposed relative to the plurality of MIDJetgenerators.